As is well known in the industrial field, in order to detect a shock on a vehicle burglar alarm or system, a Piezo sensor or a sensing unit consisting of a coil with a magnet functioning by shock, or a sensing unit consisting of a switch which would be switched on/off by vibration has been used.
In a Piezo sensor perceiving a shock there is the disadvantage that the life time of the sensor is limited due to use of a metal vibration sheet which is highly corrosive. Also, the sensitivity is not satisfactory.
On the other hand, a sensor consisting of a coil and a magnet suffers the disadvantage that a flexible suspending means such as a rubber bar must be activated by an external shock. This type of sensor is also complicated to assemble and disassemble, and also has a large volume.
Additionally, in a sensor consisting of a switch which can be switched on/off by a shock, there is the disadvantage that, due to the need for the switch contact to be activated by a shock, that sensitivity is not satisfactory.
Applicant has previously provided an enhanced sensitivity shock sensor. The sensor includes a cap that is enclosed on its upper part and is open on its bottom. A vibration sensing element such as a condenser microphone is inserted and fixed in the bottom of the cap. A sealed cavity is provided between the cap and the vibration sensing element. A rounded edge for attaching a metallic balance weight protrudes from one end of the cap. This embodiment shows the advantage of enhanced anti-corrosivity due to the sealed construction. Also, due to its simple construction, manufacturing cost is lower. Also, because vibration inside the sealed cavity is induced in proportion to the magnitude of the shock by the balance weight, as well as due to the small and light construction of the system, the design can be simplified.
Output from the above-described enhanced sensitivity shock sensor is amplified by an amplifier and supplied to an alarm system, and thereafter functions as a sensor. However, this device has a disadvantage in that peripherals such as a rubber cap is apt to be damaged by tools during assembling and setting up, and by rough handling. Also, the cap could be weakened by long term exposure to undesirable atmospheres such as toxic and corrosive gas and/or temperature and humidity.
Also, the above prior art shock sensor could be activated by air flows which come from either vehicle crossings over different directions or from inside the vehicle when parked in a sunny place or from opening of the doors. These are disadvantages since the sensor should be activated only by shocks.
The prior art has also employed bonding to join a balance weight to a cap. Use of bonding to join the balance weight to the cap may cause problems such as undesirable movement of the balance weight under conditions of high temperature and high humidity so that the reliability could not be guaranteed.